Overall : WoW : 3552 hours on main character

For consoles : skyrim (ps3) : 246 hours including dlc

SSBM for local MP.
WoW for online MP.

For single player I have no idea. CM 97/98, Outlaws, Red Alert, Heroes 3, Zelda:OoT, Gangsters, Ni no Kuni, Demons Souls, Dark Souls, GTA, GTA3, San Andreas, Persona 4 Golden, +++

Age of Empire 2, Age of Mythology, Settlers 3

Gears of War 3, over 600
and halo 3 and fable 2, both around 300 hours

1. Pokemon Diamond Edition: 620 h

2. Sonic Adventure 2: Battle (Gamecube): 198h

Counter Strike source - 908 hours on record. Garry's mod is close at 875.

Quake III~800 hours
Smash Bros Melee~650 hours
Counter Strike  1.6~600 hours
Unreal Tournament 2004~500 hours
Pokemon FireRed~400 hours
Paper Mario TTYD~350 hours
Left 4 Dead 2~350 hours
Final Fantasy IX~300 hours
Mario Kart DS~250 hours
CoD4: MW~200 hours

If we're not talking MMORPGS then most likely Tales of Symphonia or Vesperia. I always put easily 200-300 hours in tales games simply just rerunning the games main story line over and over.

If we're talking MMORPGS then I've put like 2000 hours into the ones i've played easily.

Crusader kings 2, more then 1500 hours ^^

I've spent the most time with the Total War series. Reaching 200h on Shogun 2 and Medieval 2 and already over 150h with Rome 2. Empire Total War and Napoleon combined are at 100h.

As a child however I did play a lot of Need for Speed Underground 2, C&C Tiberian Sun, Generals, Age of Mythology, Age of Empires 2 and Rollercoaster Tycoon 2 but I have no idea how many hours I've spent with those games. Could be around 100-200h each as well.

Killzone 2, Killzone 3, Battlefield 3 I have at 150h each. Uncharted 2 and 3 around 75h each. GT5 as well I guess.
Pokemon Silver and Ruby around 60h each (I remember the savedata and usually played it twice)

The next game that I'll get to the 100h mark will definitely be The Last of Us (PS3 and PS4).

Most played genre: RTS
Favorite genre: RTS + Action Adventure

CoD:MW2. sooo many hours.
Killzone 2. played it for months.
Uncharted 3. over 100 hours.